This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Our collaborators have previously isolated from tissue extracts a distinct and abundant mammalian Pol II subpopulation that contains an additional tightly associated polypeptide, Gdown1. Their results established that Gdown1-containing Pol II, designated Pol II(G), is selectively dependent on and responsive to Mediator. We are currently carrying out a thorough mass spectrometric analysis of proteins that interact with Gdown1